The contacting of solid particles and fluids (vapor, gas and/or liquid) is a relatively common operation in chemical engineering practice. It is used in adsorption and desorption processes and in variants thereof wherein thermal and/or catalytic conversions are effected. The time of contact between the solid particles and the fluid is one of a number of operating parameters which is receiving attention since the characteristics of related and/or associated chemical and/or physical change and the size of the contacting equipment is affected by contacting time. Another operating parameter of interest is the efficiency of contacting of the solids with the fluid since this, too, can affect important process and equipment features. In processes involving contacting of fluid with a fluidized bed of the solid particles, the particles adjacent to the lateral containing wall of the bed tend not to be as effective in fluid contacting as those remote from the lateral containing wall and this can lead to excessive solids-fluid contact remote from the wall in order to achieve a desired average degree of solids-fluid contacting. In the instance of fluidized catalytic cracking of a hydrocarbon feed, an under utilization of catalyst particles adjacent to the lateral containing wall and an excessive contacting of hydrocarbon feed and cracked products with catalyst remote from the wall tends to result in insufficient cracking of some feed and over-cracking of other feed so that the overall efficiency of the catalytic cracking process is lower than might otherwise be possible.
It is an object of the present invention to provide improved contacting of solid particles and fluid with a view to ameliorating at least some of the foregoing deficiencies. It is a further object of the invention to provide such improved contacting which can be implemented in a reactor for effecting contacting between solid particles and a fluid, and in particular (but not exclusively) in a reactor forming part of a fluidized catalytic cracking unit.
UK patent specification GB-A-859246 describes and claims apparatus for the fluidized catalytic cracking of hydrocarbons comprising a lower tubular fluidization section, an outwardly diverging conical transition section above the fluidization section having an angle of divergence within the range of 0.5.degree. to 2.5.degree. and a tubular cracking section above the transition section, the total length-to-average diameter ratio of the transition section and the cracking section being within the range of 10:1 to 20:1, first conduit means at the bottom of the fluidization section for injecting a gaseous fluidizing medium therein, second conduit means downstream from the first conduit means, for introducing finely divided solids into the fluidization section, and injection means downstream from the second conduit means, for injecting a liquid feed stock into the fluidization section. The top diameter of the transition section is preferably from 2 to 3 times the bottom diameter thereof.
The apparatus of GB-A-859246 thus comprises a zone for the contacting of petroleum hydrocarbons (preferably gas oil fractions boiling in the range 650.degree.-1050.degree. F. (343.3.degree.-565.5.degree. C.) and having at least 30 vol. of components boiling above 800.degree. F. (426.7.degree. C.) with finely divided cracking catalyst, and which zone comprises a first cylindrical vertical riser surmounted by a vertical divergent section connected at its top end to a second vertical cylindrical riser. The diameter of the second cylindrical riser is from 2 to 3 times that of the first cylindrical riser. The hydrocarbon feed is passed into the first cylindrical riser from a plurality of injection nozzles, and the nozzles are depicted as being at an angle of about 45.degree. to the common vertical axis of the risers. The divergent section is located a relatively considerable distance downstream of (i.e., above) the plane at which the feed enters the first cylindrical riser. It is disclosed that a suspension of catalyst in gaseous hydrocarbons is formed in the first riser wherein a conversion reaction is initiated, and during the subsequent upward flow through the divergent section, the flow velocity increases progressively to inhibit retrograde movement of the fluidized catalyst relative to the wall of the divergent section. The vaporized hydrocarbon and suspended catalyst are accelerated to an extent sufficient to provide for a disperse phase suspension of catalyst in vaporized hydrocarbon components at the top of the divergent section.
UK patent specification GB-A 1007248 describes and claims a process for cracking hydrocarbons which comprises passing a suspension of catalyst particles in hydrocarbon vapors as a substantially upflowing stream through a substantially vertically elongated conversion zone, separating spent catalyst particles from cracked hydrocarbon vapors leaving said conversion zone, passing the spent catalyst particles to the upper portion of a substantially vertically elongated separate stripping zone, through sealing means to prevent backflow of vapors from said stripping zone to said conversion zone, passing stripping gas upwardly counter current to downflowing catalyst particles in said stripping zone, removing gaseous material overhead from said stripping zone, withdrawing dense fluidized stripped catalyst particles through a standpipe from the bottom portion of said stripping zone and passing them to the lower portion of a regeneration zone, passing regenerating gas upwardly through the catalyst particles to maintain a dense fluidized bed of catalyst particles while regenerating them, withdrawing regenerated catalyst particles from the lower portion of said regeneration zone and passing them to the lower portion of said elongated conversion zone and introducing hydrocarbon oil into the lower portion of said conversion zone for admixture with the introduced regenerated catalyst particles to form said suspension of catalyst particles above mentioned and circulating solids from said stripping zone to said regeneration zone by maintaining a higher pressure of catalyst particles in the bottom portion of said stripping zone than in the lower portion of said regeneration zone. In the illustrated embodiment, the substantially vertically elongated conversion zone comprises a relatively long, hollow cylindrical reactor, having the form of a relatively wide pipe, which surmounts a narrower, relatively short tube described as the injection section. Hot regenerated catalyst particles are received at the bottom region of the injection section, and they are entrained upwardly into the reactor by steam which is injected at the base of the injection section. Feed which is to be cracked is injected into the bottom portion of the reactor for admixture with the upflowing suspension of catalyst particles in steam from the injection section. The oil feed is atomized and vaporized and mixed with the catalyst particles so that the suspension of catalyst particles in oil vapor passes upwardly in the reactor at a velocity between about 3 and 12 m/s. At spaced elevations along the interior of the reactor, there are provided venturi shaped contacting devices. The venturi contacting devices ensure co-current contacting between the solid and vaporous materials, and in particular, the mainly solids containing stream which passes downwardly along the walls and the main stream or suspension of solid catalyst passing up through the center of the reactor. There is no disclosure or suggestion in GB-A-1007248 of contacting droplets of oil feed and catalyst particles in any of the venturis or in any particular region of any of them.